When operating at 100 ° C, the luminous efficiency is only reduced by about 3%
It is required that the white LED has "temperature stability" in order to make the luminous efficiency not greatly change under actual use environment.
The luminous efficiency of the white LED directory value is mostly measured by applying a pulse current at an ambient temperature of 25 °C. At this time, the temperature (joining temperature) of the light-emitting portion of the blue LED chip was about 25 °C. However, when actually used in a product, the bonding temperature rises to 85 to 100 °C. In ordinary white LEDs, the luminous efficiency after temperature rise is reduced by 10 to 15% compared with that at 25 °C.
Among some of the latest products, there have been varieties that suppress changes in luminous efficiency. For example, Philips Lumens' white LEDs for lighting use have a luminous efficiency that is only about 3% lower than that at 25 °C at a junction temperature of 100 °C. In order to improve heat dissipation, the LED chip is flip-chip packaged on a ceramic substrate, and a quantum well structure or the like is used to reduce the influence of temperature on the internal quantum efficiency.
Sharp's white LEDs for lighting, which began shipping in March 2011, have a relatively small change in luminous efficiency with temperature (Fig. 9). Although the input power is as high as 25 W, it is difficult to lower the bonding temperature. However, when the white LED surface temperature (case temperature) is 70 ° C, the luminous efficiency and luminous flux are only reduced by 5 to 6 percentage points compared with the case of 25 ° C. The company said it will control the decline to about half compared to other companies' equivalent products. Higher temperature stability is achieved by using a fluorescent material whose performance changes little with temperature.
Figure 9: Temperature dependence of weakening the performance of fluorescent materials Sharp white LEDs (products with an input power of 25 W) developed and supplied for lighting applications have a low temperature dependence on luminous efficiency. The company said it uses a fluorescent material with a small change in luminous efficiency with temperature. This reduces the decrease in brightness. (Photo: Sharp)
Except for visible light, the "color rendering index" is an important indicator based on the wide application of white LEDs as a general lighting source. The purpose is to not create a sense of discomfort with the original light source. Previously, the color rendering index of white LEDs was mostly based on the average color rendering index (Ra). However, as the situation for lighting applications increases, only Ra is no longer satisfactory, and a higher special color rendering index, such as red and green with higher chroma, is required.
The color rendering index is improved by improving the fluorescent material. However, increasing the color rendering index also causes a problem of reduced luminous efficiency. For example, assuming that the ordinary white LED with Ra of 70 has a luminous efficiency of 100%, it will decrease by 10% in Ra80 and by 20% in Ra90. “In the past, when Ra was above 90, the luminous efficiency was reduced by 30%†(Nichia Chemical Industry). Therefore, compared with before, it has improved a lot now, but there is still much room for improvement in the future.
The focus of the improvement is to “reduce luminescence outside the visible range†(Philips Lumen in Japan) (Figure 10). In general, the white LED will add a red fluorescent material to increase the Ra and red color rendering index (R9), and the red fluorescent material can reach the near-infrared field of 700 nm or more. Since the light outside the visible range is invisible to the naked eye, all of this energy is wasted. So the company used a red fluorescent material that reduced this part of the light.
Fig. 10: Reduction of luminescence outside the visible range When a red fluorescent material is used, luminescence can be observed also on the long wavelength side of 700 nm or more. This kind of light outside the visible range is not visible to the naked eye, and thus wastes energy efficiency. Therefore, by using a red fluorescent material that suppresses light emission outside the visible range, it is possible to suppress a decrease in luminous efficiency.
In order to balance the high color rendering index and luminous efficiency, there are also manufacturers considering the use of non-polar GaN substrates. In order to improve Ra and special color rendering index, Mitsubishi Chemical did not use blue LED chips, but used purple LED chips to develop white LEDs using blue, green and red fluorescent materials (Figure 11). The company plans to use a non-polar GaN substrate to significantly increase the luminous efficiency of only about 50 lm / W. 1).
Note 1) Mitsubishi Chemical has participated in the project of the Japan New Energy Industry Technology Development Organization (NEDO), "Basic Technology Development to Improve the Efficiency and Quality of LED Lighting", and is developing GaN that can realize high efficiency and high color rendering white LEDs. Substrate.
Figure 11: Achieving High Chromogenic Index Characteristics Mitsubishi Chemical's combination of white LEDs with a purple LED (luminous wavelength of 405 nm) and blue, green and red fluorescent materials can greatly increase the color rendering index of each color. (Photographed by the site based on Mitsubishi Chemical's speech on "Green Device Forum 2010")
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